PROJECT SUMMARY Alzheimer's Disease (AD) affects nearly six million Americans and is the sixth leading cause of death in the United States. The number of patients is growing rapidly, with an enormous burden on the healthcare system ? it is estimated that early and accurate diagnosis could save up to $7.9 trillion. The consequences of neurodegeneration in AD and many other incurable disorders are devastating, with no treatments that can halt or even reduce their progression. Mouse models of AD have been the preferred system for discovering candidate therapies for AD and related disorders, but all of these therapies have failed in human clinical trials, likely owing to important differences between mouse and human neurobiology that have recently come to light. Overcoming the dearth of effective therapeutic strategies requires approaches that will allow researchers to thoroughly investigate the mechanisms that drive neurodegeneration in humans. Recent findings suggest that astrocytes, star-shaped cells that make up 50% of the human brain and spinal cord, can sometimes malfunction in ways that make them toxic to neurons, which is a plausible path to neurodegeneration ? but these findings were in the mouse model, and it is not yet clear what effects these malfunctioning astrocytes have in human brains. Advancements in induced pluripotent stem cell technology have made patient-derived brain cells accessible for disease research, requiring only skin or blood samples rather than invasive surgeries. This project will investigate the biology of this newly discovered category of neurotoxic astrocytes using induced pluripotent stem cells to create several types of brain cells implicated in AD. The molecular differences between neurotoxic and normal astrocytes will be assessed, as well as the factors that trigger the transition from normal to neurotoxic. The nature and extent of astrocyte neurotoxicity will be determined using stem cell-derived neurons. Finally, to investigate the role that neurotoxic astrocytes may have in AD neurodegeneration, the same brain cell types will be derived from AD patients and the same datasets will be generated. This will provide preliminary indications of whether the genetic risk factors associated with AD may be exerting their harmful effects by increasing the neurotoxicity of astrocytes, relative to healthy individuals. Overall, this project will provide the first functional investigation of the recently discovered phenomenon of malfunctioning astrocyte neurotoxicity and its potential implications for AD. These studies are likely to yield novel insights into the mechanisms that drive neurodegeneration, which may point to new therapeutic strategies to prevent neuronal death in AD and a host of severe, currently incurable diseases.